The present invention relates generally to measuring load current in a switching power supply and, more particularly, to an apparatus and method for measuring DC load current of a switching power supply.
Many applications have need for accurate measurement of DC load current on the secondary side of a power transformer used in a switching power supply. For example, accurate measurement of current output to a battery in a battery charger is essential for ensuring proper charging of the battery within its particular parameters. Typically for power supplies having a switching power supply input and a high frequency step-down power transformer the output DC current on the secondary side of the power transformer is monitored by a current shunt resistor or a Hall-effect transducer wired to the output circuit in order to derive an accurate linear measurement. On high current supplies, however, current shunt resistors become large, bulky and result in power dissipation of a portion of the output power. Additionally, Hall-effect transducers are expensive and wiring such transducers into a high-current circuit, for example, poses additional design and manufacturing problems.
An alternative approach to the above methods of measuring output DC current includes monitoring the primary current of the high frequency power transformer by using an inexpensive current transformer and determining the output DC load current of the power transformer relating the current induced on a secondary side of the current transformer to the DC load current according to the turns ratios of the power and current transformers. This approach, however, becomes problematic during short-circuit conditions. In addition, the primary current contains reflected secondary DC output current as determined by the turns ratio of the power transformer added to the magnetizing current within the core of the power transformer. Also, the primary current is typically pulse-width modulated in order to regulate output voltage to the power supply. Hence, simply placing a current transformer on the primary side is problematic because simple rectification and averaging of the primary current produces a nonlinear transfer function (i.e., a function relating the measured primary current to the output load current) with respect to the actual load current as the duty cycle of the pulse-width modulation varies. Furthermore, given light loads on the secondary side of the transformer, the magnetizing current becomes large compared to the secondary current reflected to the primary and, thus, simple rectification may result in large errors.
Attempts to cure the noted problems above have included peak detection rather than simple rectification and averaging of the primary current. Again, however, measurement errors result when the duty cycle changes due to a corresponding change in the magnetizing current amplitude within the power transformer. Similarly, peak detection also suffers from large errors at light loads due to the effects of transformer magnetizing current.